I cannot find interpolate key of grease pencil in tab
I had this issue also, Make sure you are using the latest "Blender Builder" /
here is the link https://builder.blender.org/download/

I'm guessing you saw the youtube tutorial from OpenToonzTraining. you're using blender 2.78. If possible when you see his video you will see he is using 2.78.1

Me personally, I think I'll wait for blender 2.79, as there is a risk of instability with the builds as it says on top of it, but I don't know about that stuff yet.